The present invention relates generally to a transducer element for measuring angular and linear acceleration, and more specifically for use preferentially in modal testing and for measuring rotational acceleration on shafts, and an accelerometer equipped with it.
Knowledge of the vibration behavior of engineering structure is of fundamental importance in many domains. In view of its importance, analysis of the vibration behavior has recently acquired a special name: modal testing. It is partly experimental, partly theoretical in character. Its purpose is to obtain a mathematical model of the vibration behavior of a structure. The importance of this for turbine blades, but also for bridges, is obvious. Accelerometers are suitable instruments for analyzing vibration behavior. They measure accelerations that occur typically during vibration. Owing to the lack of suitable instruments, chiefly linear accelerations have hitherto been measured. This was done by joining the accelerometer rigidly to the surface of the object to be measured, either by screwing or gluing it. When vibrations occurred on the object, the accelerations could be measured by a conventional accelerometer of this kind typically at right angles to the surface, in one dimension that is. Also, two- or three-dimensional acceleration measurements were and still are used, by measuring the acceleration additionally in one or two directions at right angles to each other and parallel with the surface of the object to be tested. Nevertheless, linear accelerations have been measured previously for the most part. Yet, the theory of modal testing, as expounded for example in the book "Modal Testing, Theory and Practice" by D.E. Ewins, Research Study Press Ltd., 1984 on pages 87-152, indicates that angular accelerations are just as important as linear accelerations and that modal testing is incomplete without analyzing the angular accelerations.
In the book named, on pages 146 to 148, the principles of angular acceleration measurement are also set out, based on measurement of rotary impulses by registering acceleration with two accelerometers at a certain distance apart. In commercial printed matter, instruments are also described occasionally which serve to measure both linear and angular acceleration. The measurement of angular rate is described moreover in U.S. Pat. Nos. 2,716,893; 3,842,681 and 4,431,935. However, no instrument is described capable of measuring angular acceleration and linear acceleration together. Moreover, the accelerometers described have insufficient sensitivity and are too big and too heavy for modal testing.
It is therefore the purpose of the present invention to provide a transducer element or accelerometer of the kind mentioned at the beginning, allowing exact measurement of both angular and linear accelerations and particularly suitable for modal testing, though not solely for this.
The purpose is fulfilled by cantileverly mounting a pair of electromechanical beams to a main axis. The beams oscillate in response to vibrational acceleration of the object to which they are attached and thus will be called herein oscillating beams. When the beams are made of material having electrical polarization directions, they will be referred to as having electrical polarities. The electrical output signals, produced by the flexure, are provided to a signal processing facility, which adds the output signals if the electrical polarity of the oscillating beams is dissimilar and subtracts them if the polarity is the same to generate a characterizing measuring signal for the angular acceleration about the main axis. By reversing the operations, linear instead of angular acceleration may be measured. A signal processing facility allows simultaneous measurement of both angular and linear acceleration. The transducer element is of simple and rugged construction, permitting each miniaturization for the requirements of modal testing. Thus, both angular and linear accelerations can be detected with one and the same transducer element. According to a further development of the invention, the oscillating beams are mounted preferably by fixing elements on a base plate joinable with a housing. The oscillating beams may be piezoelectric bending elements, bending elements coated with piezoresistive semiconductor material, or bending elements provided with strain gauges. The electronic equipment may be integrated wholly or partly in the transducer element. Furthermore, the invention creates an accelerometer for measuring angular and/or linear acceleration, featuring a housing in which at least one transducer element is allotted preferably to each of the three main axes, for detecting the angular and/or linear accelerations in the three coordinate directions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.